feat: add Pastel color palette generator to Kit stack

Added Pastel service with API and UI to the Kit toolkit:

**New Services:**
- pastel_api: Backend API for color palette generation
  - Image: ghcr.io/valknarness/pastel-api:latest
  - Routes: https://pastel.kit.pivoine.art/api

- pastel_ui: Frontend UI for interactive palette generation
  - Image: ghcr.io/valknarness/pastel-ui:latest
  - Routes: https://pastel.kit.pivoine.art

**Features:**
- Color harmony algorithms
- Interactive palette generation
- Export in various formats
- Programmatic API access
- Path-based routing (UI on root, API on /api)

**Configuration:**
- Updated arty.yml with KIT_PASTEL_* variables
- Updated documentation (CLAUDE.md, README.md)
- Added Traefik labels with SSL, compression, security headers
- Watchtower auto-update enabled

Kit stack now includes 5 services:
- Landing page (kit.pivoine.art)
- Vert file converter (vert.kit.pivoine.art)
- Paint image editor (paint.kit.pivoine.art)
- Pastel color generator (pastel.kit.pivoine.art)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

arty.yml

@@ -118,6 +118,9 @@ envs:
     KIT_VERT_IMAGE: ghcr.io/vert-sh/vert:latest
     KIT_VERT_TRAEFIK_HOST: vert.kit.pivoine.art
     KIT_PAINT_TRAEFIK_HOST: paint.kit.pivoine.art
+    KIT_PASTEL_API_IMAGE: ghcr.io/valknarness/pastel-api:latest
+    KIT_PASTEL_UI_IMAGE: ghcr.io/valknarness/pastel-ui:latest
+    KIT_PASTEL_TRAEFIK_HOST: pastel.kit.pivoine.art
     # Jellyfin
     JELLY_TRAEFIK_ENABLED: true
     JELLY_COMPOSE_PROJECT_NAME: jelly